This invention relates to carbon fibers and more particularly to continuous pitch-based carbon fibers having a high modulus and low electrical resistivity and methods for the production of such fibers, and to composites comprising such fibers.
Carbon fibers have long been known, and methods for their production from a variety of precursors are well described in the art. Cellulosic precursors have been used for producing carbon fiber since the early 1960's, with rayon being the dominant carbon fiber precursor for nearly two decades. More recently, as the art has developed methods for producing carbon fiber derived from such materials as polyacrylonitrile (PAN) and pitch, the importance of rayon-based carbon fiber has declined. This shift has been due in part to the superior toughness, tensile strength and stiffness exhibited by both PAN-based and pitch-based carbon fiber. In addition, the conversion yield of rayon to carbon fiber is low, and the resulting carbon fiber is ordinarily lower in density than carbon fiber based on PAN or pitch, which further limits its potential uses.
It is known that the tensile modulus of carbon fiber generally increases with increasing density, as does the thermal conductivity, while the electrical resistivity of carbon fiber decreases as fiber density is increased. Carbon fiber with high thermal conductivity has found use in applications where heat dissipation is a requirement such as, for example, in the manufacture of heat sinks and in brake pad applications, while fiber with a high degree of stiffness lends greater dimensional stability to composites. Considerable effort has therefore been expended to achieve carbon fibers with these high densities reproducibly and with good control.
Polyacrylonitrile fiber, when oxidized and carbonized under appropriate conditions, provides tough, high strength, high modulus carbon fiber. The overall conversion yield in producing fiber from PAN is good, and the finished fiber is capable of achieving the outstanding tensile strength needed for producing the high performance composite materials used in a variety of sports, automotive and aircraft applications. However, the tensile modulus of commercially available PAN-based fiber does not generally exceed about 50.times.10.sup.6 psi, which is somewhat deficient for use in applications that require a high degree of stiffness. Moreover, PAN-based carbon fibers generally exhibit densities of less than 1.9, together with low thermal conductivity, ordinarily less than 200 w/m-.degree.K, and high electrical resistivity.
Pitch-based carbon fiber has generally been recognized as capable of providing greater stiffness and higher thermal conductivity than carbon fiber from other sources, and considerable effort has been directed toward the development of pitch-based ultra-high modulus carbon fibers with good thermal conductivity. Such carbon fibers could find immediate application in forming composites for use where good dissipation of electrical charges or heat is desired. In addition, the combination of high stiffness and good thermal ccnductivity with the negative coefficient of thermal expansion characteristically exhibited by pitchbased fibers would make such composites extraordinarily dimensionally stable.
The continuous carbon fibers heretofore disclosed and described in the art, including those carbon fibers having tensile modulus values as great as about 120 to 125.times.10.sup.6 psi which have been designated as "ultra-high modulus", have generally exhibited densities of less than about 2.2 g/cc, thermal conductivities of less than about 1000 w/m-.degree.K and electrical resistivities generally above about 1.8 micro-ohm-meter. For most high modulus, pitch-based carbon fibers produced in commercial facilities, the thermal conductivity ordinarily falls below about 700 w/m-.degree.K, and the electrical resistivity is generally above 2.0 microohm-meter. Although there has recently been reported in the art pitch-based carbon fiber having a tensile modulus substantially above about 125.times.10.sup.6 psi, with single carbon fiber filament values as great 140.times.10.sup.6 psi, these fibers also generally do not exhibit low electrical resistivity characteristics, and the thermal conductivity of these fibers is also reported to be low, generally below 1000 w/m-.degree.K.
Crystalline graphite has a density of about 2.26 g/cc, and generally exhibits excellent thermal conductivity, near 1800 w/m-.degree.K, and low electrical resistivity, well below 1.5 micro-ohm-meter. However, even though methods for producing graphite whiskers having extremely high modulus together with conductivity and density properties near those of single graphite crystals are known, the art has not suggested the preparation of continuous carbon fibers from pitch or any other source with a density of 2.2 g/cc or greater, a thermal conductivity well above 1100 w/m-.degree.K and an electrical resistivity significantly below 1.5 micro-ohm-meter, to as low as 1.2 micro-ohm-meter and lower.
A carbon fiber having a density of about 2.2 or greater and an electrical resistivity below 1.5 micro-ohm-meter, together with a tensile modulus well above 125.times.10.sup.6 psi and even as great as 130.times.10.sup.6 psi or greater would be a substantial advance in the carbon fiber art. Such carbon fiber, and particularly fiber exhibiting a thermal conductivity greater than 1100 w/m-.degree.K, would find immediate wide acceptance for use in a variety of composite applications, and would be particularly useful for composites in which good dimensional stability and low electrical resistivity are needed.